1. Field of the Invention
The invention is directed to a magnetic core for an interface transformer. More specifically, the invention is directed to an interface transformer having a core material which enables the transformer to be utilized in an S.sub.o interface of an ISDN network, such a transformer being employed at the interface between the network termination and the individual terminal equipment.
2. Description of the Art
An integrated services digital network (ISDN) is a recently developed worldwide digital communications system. In such a network, a Uk.sub.0 line interface provides the required connection between a digital local switching center and a network termination. The distance between the digital, local switching center and a network termination in such a system can amount to a maximum of 8 km.
Up to eight terminal units can be connected to a single network termination. The terminal units, for example, can be telephones, picture screen telephones, picture screen text, facsimile, textfax, work station, etc. The terminal units can be located at a distance of up to 150 m from the respective network termination.
The interface between the network termination and the terminal unit is referred to as an S.sub.o user interface. The various electrical characteristic requirements of such an S.sub.o interface are defined in the international standard CCITT I.430 or, respectively, in the Standard FTZ 1 TR 230 of the German Federal Mails. These standards define, inter alia, the impedance of the interface as a function of the frequency as well as the pulse mask for the transmitted, digital pulses.
A company publication, PUBL 1101 E by H. Hemphill, Using Pulse Transformers for ISDN-Applications, Schaffner Elektronik AG, Luterbach, Switzerland, is concerned with the magnetic and electrical property requirements of S.sub.o interface transformers that are based on these standards. For example, FIGS. 2 and 3 in this publication set forth the impedance and pulse transmission requirements according to the postal standards.
Among the specific items discussed in this publication are RM6 cores which are used as magnetic cores for S.sub.o interface transformers. Ferrite is cited as the core material. Use of such ferrite cores limits the values for the permeability .mu. and the saturation induction Bs. Typical values for these variables are .mu.=10,000, Bs=0.45 T (SIFERIT T38 of Siemens).
Whether a digital pulse can be transmitted within the prescribed pulse mask is essentially dependent on the inductance and capacitance characteristics of the transformer. The inductance L of the transformer dictates the pulse droop of the transmitted pulse. The pulse droop is defined as the undesired decrease of the voltage which the transmitted pulse experiences during the course of the pulse duration. In order to satisfy the ISDN demands with respect to the pulse droop values, the inductance of the transformer must be greater than 20 mH at 10 kHz.
The coupling capacitance values of a transformer also define the signal shape of the transmitted pulse. This coupling capacitance is the capacitance between two different windings of the transformer and is dependent, inter alia, on the number of applied turns as well as on the winding arrangement. In particular, this coupling capacitance defines the shape of the pulse as it makes the transition from its high status to its low status. To maintain the integrity of the pulse shape, the transformer is designed so that the coupling capacitance is minimized.
The inductance of the transformer is directly proportional to the permeability of the core material. In order to satisfy the ISDN demands with respect to the inductance, particularly given a DC pre-magnetization of the transformer, a comparatively large magnetic core cross-section is required. A larger magnetic core cross-section, however, means enlarging the magnetic core and, thus, enlarging the structural volume of the transformer. Optimally small components are desirable. Alternatively, the ISDN demands with respect to the inductance may be accomplished with a larger number of turns of the transformer winding. A higher number of turns, however, results in an increase in the coupling capacitance and, thus, a deterioration of the transmission behavior. The increased capacitance due to the added turns can be partially overcome by utilizing complicated winding arrangements having insulating layers lying between the windings. This complicates the manufacture of the winding and, thus, renders such transformers more costly.